This invention relates to a ceramic matrix composite and to a method of decreasing the brittleness of ceramic matrix composites. More specifically, this invention is directed to a ceramic matrix composite having a potential debonding interphase and to a method of strengthening ceramic matrix composites by lattice deformational transformations of interphases.
The brittleness and unreliability of ceramics in certain applications continue to present difficult and unsolved problems. The aerospace, automotive and aviation industries are but a few examples of industries that are searching for enabling technology to introduce new types of ceramics that are tough, flaw tolerant and exhibit graceful failure and creep resistance for both ambient and high temperature applications. Typical applications include components in turbine engines, cylinder sleeves for gasoline engines and structural components.
Recent trends in the ceramics research have been to reinforce brittle ceramic matrices with higher elastic modulus fibers, platelets, particulates, or whisker-shaped reinforcing elements embedded in the matrix. The reinforcing elements impart additional strength to the ceramic matrix. The additional strength is necessary to maintain the structural integrity of the ceramic matrices, particularly upon stress or shear-induced defects. These embedded reinforcing elements constitute large amounts of interfacial surface inside the ceramic composite. The deflection of a crack along such an interface causes separation of the interface due to the action of an impinging crack and is an important mechanism for enhancing the fracture toughness of the ceramic matrices. In a fiber-reinforced matrix, for example, the advancing crack can directly advance through the fiber potentially destroying the ceramic or it can debond along the interface and inhibit fiber failure. If debonding occurs, then the intact fibers will allow crack bridging and eventual fiber pull-out, thus giving rise to increased toughness of the composite.
It has been demonstrated that extensive fiber pullout can be induced by formation of a weak interphase layer between the fiber and the ceramic matrix. This has led to investigations of a variety of composites with coated fibers. This type of interfacial debonding mechanism of toughening has clearly been demonstrated in SiC materials reinforced with SiC fibers that were previously coated with a thin layer of compliant graphite (C) or boron nitride (BN). Toughness values of up to 30 MPa mxc2xd have been reported for the graphite system when it operates under vacuum and ambient temperature. Alternating laminates of silicon carbide (SiC) and graphite have also functioned well in controlled oxygen deficient atmospheres. However, for high temperature applications (greater than 1200xc2x0 C.), for extended use (greater than ten hours), or in air or oxidizing environments, both silicon carbide and graphite are chemically unstable and decompose to silica (SiO2) and gaseous species, e.g., carbon monoxide (CO). The resulting ceramic body is left porous, friable and weak.
An alternative system that has been proposed to replace carbon or boron nitride coatings around oxide fibers is based on micaeous cleavage material such as fluorophlogopite or xcex2 alumina/magnetoplumbite. More recently, lanthanide phosphates (LnPO4) type compounds such as monazites (e.g. LaPO4 or zenotimes such as yttrium phosphate (YPO4) have been introduced as oxidation resistant weak interfaces capable of operating up to 2000xc2x0 C. in air.
One shortcoming of the interface slippage mechanism operating in the monazite-type systems is that it relies on the fiber preserving a smooth and clean interface. This places a stringent requirement on the manufacture and handling of the fiber and hard-to-control restrictions on its subsequent change in surface microstructure during prolonged service at high temperatures.
Current methodologies to reduce the brittleness of ceramics also include transformation toughening of ceramic composites by providing an interphase material embedded in the ceramic composite that undergoes a physical change upon mechanically induced stress. One such system investigated includes zirconia (ZrO2). Zirconia undergoes a tetragonal to monoclinic phase transformation. The tetragonal to monoclinic phase transformation for zirconia is considered to be martensitic in bulk and is accompanied by a volume increase of about 3% at 950xc2x0 C. and about 4.9% at room temperature. The transformation is notorious for causing unstabilized zirconia to shatter. However, these properties can be utilized to enhance the mechanical properties of a ceramic matrix that includes zirconium oxide particles.
It is known that a fine-grain material, for example, alumina, can be toughened by dispersions of randomly oriented intergranular, irregularly shaped zirconium oxide particles (ZTA). The mechanism of transformation toughening is attributed to energy dissipation of the propagating crack by the tetragonal to monoclinic transformation in the crack tip stress field, followed by the exertion of closure forces on the crack resulting from the volume expansion experienced by transformed ZrO2 particles lying in the transformation zone of the crack. Mechanical properties of zirconium oxide, such as toughness and thermal shock resistance, are significantly improved and fracture of the material is retarded. However, use of zirconium oxide particles embedded in ceramic matrixes only yields about three to four-fold improvement in the strength of ceramics. Much greater strength improvements are required before ceramics can replace alloys currently used in the aerospace, aviation and automotive industries. Thus, there remains a need of less brittle ceramic composites that can be used to replace the metals and alloys currently used.
Thus, there is provided in accordance with the present invention a ceramic composite. The composite comprises a ceramic matrix that includes at least a first ceramic material; the composite also comprises a second phase, which includes second material, and an interphase material. The interphase material is positioned between the ceramic matrix and the second phase material. The interphase material includes a metastable ceramic oxide that is compositionally stable under the processing conditions to form the ceramic material. Preferably, the metastable ceramic is compositionally stable at a temperature of from at least about room temperature to about 2000xc2x0 C. In one embodiment, the metastable ceramic includes xcex2-cristobalite. The metastable ceramic is capable of undergoing a stress induced, zero volume or negative volume transformation, which may or may not be accompanied by a unit cell shape change.
There is also provided in the present invention a method of preparing a toughened ceramic composite. The method comprises bonding a first ceramic material to a transformable interphase material. The transformable material is capable of undergoing a martensitic, zero volume or negative volume, unit cell shape change under shear or stress conditions. The transformation may or may not be accompanied by a unit cell shape change. A second phase material also is bonded to the interphase material. The second phase material can be a ceramic such as the first ceramic material to provide a laminate or the second phase material can be provided as a reinforcing element (e.g., fiber, platelet) in the ceramic composite. The ceramic composite can also include a second interphase material in addition to the first ceramic material, the transformable interphase material and the second phase material. The second interphase material includes a transformable material that is capable of undergoing a positive volume change, martensitic phase transformation under shear or stress conditions.
In another aspect of the invention there is provided a method of enhancing the strength of a ceramic composite by forming an interphase material between a first ceramic matrix and a second phase material. The interphase material comprises a metastable ceramic oxide that is capable of generating stress induced microcracks within the interphase material. The interphase material deflects a stress default in the ceramic composite by a zero volume or negative volume phase transformation, which may or may not be accompanied by a unit cell shape change.